FIG. 1 is a schematic diagram of a typical prior art laser 2 employing a traditional acousto-optic Q-switch 10a. FIGS. 2A and 2B (generically, FIG. 2) are alternative partly schematic views of a prior art acousto-optic modulator (AOM) 10 having a transducer 12 responsive to a radio frequency (RF) driver 14 that controls the extent to which the AOM 10 transmits a zero-order beam 16 and/or a first-order beam 18. FIG. 3 is a schematic view showing the traditional technique for controlling the RF driver 14. With reference to FIGS. 1–3, AOMs 10 have traditionally been used as Q-switches 10a within the resonators of lasers 2 to control pulse timing, repetition rate, and cavity gain. A typical Q-switch 10a (or a typical AOM 10) includes an RF transducer 12 that is amplitude-modulated by the RF driver 14 at a specific frequency set by a manufacturer. The Q-switch 10a is typically controlled by a laser system controller 4 that commands a power supply 14a to provide a selectable amount of power to the RF transducer 12 to allow laser pulses to either exit the laser or to hold the laser energy inside the laser resonator. The power supply 14a also typically provides power to a laser pumping source 6 to provide pumping radiation to a laser medium 8 in response to commands from the laser system controller 4. These components cooperate to produce a pulsed laser beam 20 when desired.
AOMS 10 have also been used as variable intra-resonator loss modulators to control laser pulse timing and intensity by variably controlling the amplitude of the RF signal delivered to the RF transducer(s) 12 on the AOM(s) 10 as described in U.S. Pat. No. 5,197,074 of Emmons, Jr. et al. AOMs 10 have also been used as extra-cavity beam attenuators that control the intensity of the laser beam 20 by diffracting the laser beam 20 with varied diffraction efficiency so that a percentage of the optical energy travels down a desired beam path and most of the rest of the optical energy travels to a “beam dump.”
More recently, Electro Scientific Industries, Inc. of Portland, Oreg. has employed AOMs 10 as gating control devices or “pulse pickers” to allow pulses from a laser 2 to propagate through or along various positioning system components to impinge a workpiece when commanded and to inhibit the laser pulses from impinging the workpiece when not commanded. This process is described in more detail in U.S. Pat. No. 6,172,325 of Baird et al.
With reference again to FIGS. 2 and 3, the transducer 12 converts an RF input signal from the analog RF driver 14 into a sound wave 19 that sets up in the AOM 10. As the sound wave 19 transverses through the AOM 10, the sound wave 19 distorts the optical media of the AOM 10, causing increases and decreases in indexes of refraction in the AOM 10. Thus, an incoming laser beam 20 is diffracted by the sound wave 19 and follows the laws of diffraction, resulting in the zero-order beam 16 that is on-axis and in first-order (or higher-order) beams 18 at angles specified by equations relating to the diffraction process.
When no RF power 22 is applied to the AOM 10, the incoming laser beam 20 passes through the AOM 10 substantially along its original beam path. When the RF power 22 is applied to the AOM 10, part of the incoming laser beam's energy is diffracted from the beam path of the zero-order beam 16 to a beam path of a first-order beam 18. The diffraction efficiency is defined as the ratio of the laser energy in the first-order beam 18 to the laser energy in the incoming laser beam 20.
With reference to FIG. 4, either the first-order beam 18 or the zero-order beam 16 can be used as a working beam to impinge a workpiece 30, based on different application considerations. When the first-order beam 18 is used as the working beam, the energy of the working laser pulses can be dynamically controlled from 100% of its maximum value down to substantially zero, as the RF power 22 changes from its maximum power to substantially zero, respectively. Because the practical limited diffraction efficiency of an AOM 10 under an allowed maximum RF power load is about 75% to 90%, the maximum energy value of the working laser pulses is about 75% to 90% of the laser pulse energy value from the laser.
However, when the zero-order beam 16 is used as the working beam, the energy of the working laser pulses can be dynamically controlled from about 100% (minus losses from traveling through the AOM 10, perhaps as much as a few percent due to thermal and dispersion considerations) of the maximum value of the laser pulse energy from the laser down to about 15% to 20% of the maximum value, as the RF power 22 changes from substantially zero to its maximum power, respectively. For memory link processing, for example, when the working laser pulse is not on demand, no leakage of system laser pulse energy is desired (i.e., the working laser pulse energy should be zero), so, as shown in FIG. 4, the first-order beam 18 is used as the working beam and the zero-order beam 16 is directed to a beam dump, such as an absorber 32.
An extinction ratio 34 of the AOM 10 defines the difference in transmitted power of a laser pulse 36 (36a or 36b) between an “unblocked” (or “transmitting”) state 38 and a “blocked” or “nontransmitting” state 40. FIG. 5 is a simplified generic graph showing the differences in transmittance of blocked and unblocked laser beam 20 as a function of decibel (dB) level applied to the AOM 10 at a specific frequency. With reference to FIGS. 3 and 5, conventional AOMs 10 used in pulse-picking laser systems receive, from a constant frequency generator 24 (typically a PLL or a crystal), a specific single radio frequency that is set by a manufacturer and cannot be changed. This frequency determines the output angle and controls the amount of diffraction by RF amplitude within the limits of the extinction ratio 34.
The amplitudes of the signals sent to the analog RF drivers 14 of conventional AOMs 10 can be controlled by either sending a transistor-transistor logic (TTL) “ON” or “OFF” signal from an on/off digital controller 26, and/or by sending an analog signal of 0–1 volt in non-integer increments from an analog amplitude control board 28, into the RF driver 14. The TTL “OFF” signal directs the analog RF driver 14 to lower the output to the minimum level, which is the lowest power output the RF driver 14 will allow. Setting the analog signal into the RF driver 14 at its minimum level will accomplish the same result. Both of these options will, however, still allow the transmission of a small amount of RF power 22 to transducer 12, creating a low-energy diffracted first-order beam 18 that passes to the workpiece 30 when it is not wanted.
As laser powers continue to increase for a variety of laser applications (such as laser DRAM processing, laser trimming and micromachining, and laser micro-via drilling), many of these laser applications seek the ability to turn completely off the laser power to the work surface. In these laser operations, the workpiece may be expensive in terms of materials and/or prior processing. If the laser output does not turn completely off, there is potential to “leak” or diffract energy to the workpiece in locations where damage to, change to, or effect on the material properties or characteristics is unacceptable. In laser trimming, unwanted energy could, for example, induce undesirable electro-optical effects in the material that are undesirable. Regardless of the laser operation, leaked laser energy has the potential to cause significant irreversible damage to a customer's product, such as devices on a wafer, and such damage may not necessarily be noticeable by visual inspection. Energy leakage problems in laser systems can occur in a continuous range of wavelengths, including long-wavelength CO2 (about 10 μm), infrared and near infrared (such as 1.3 μm to 1.0 μm), visible, and UV (less than about 400 μm).
With the increasing use of AOMs 10 in laser processing applications, energy leakage problems become increasingly more evident. Unfortunately, even when the minimum RF level is sent to state-of-the-art AOM controllers, there is still some RF power 22 that leaks into the AOM 10, causing some amount of laser beam energy to be diffracted to a potentially unwanted location. Such leakage can also occur when traditional Q-switches 10a are employed, allowing for some laser energy to exit the laser 2 during the laser energy buildup time when laser output is undesirable.